Strange UIWebView memory usage with many animations-Collection of common programming errors

I am using UIWebView to display a page with tons of animations (around ~40 webkit keyframe animation, each of them with 40~70 keyframes). Most of the animations loops infinitely. The problem is, the app will crash around 30 seconds later after the page is displayed. And I tested that the app would crash in iPad 3 but not iPad 2. More strangely, I profiled the app and got this result:

The app’s free memory dropped suddenly (from 5xx MB to 1xx MB) within 5 seconds. But the used memory did not rise.

The mostly increased memory came from IOKit, which allocated 100+ MB within the last 5 seconds. I don’t know why this is the case.

The problem is: why the used memory does not increase while the free memory decreases dramatically?